The disclosed invention relates to the homodyne processor for use in a sonar system. Specifically, the disclosed invention is directed to a homodyne processor which utilizes digital circuitry to perform real-time detection and analysis of received signals which are caused by sonar reflection of a transmitted waveform.
Homodyne circuits in general are utilized for quadrature processing in sonar systems, as exemplified in U.S. Pat. No. 4,119,940 issued to Keating et al on Oct. 10, 1978.
The theoretical aspects of quadrature processing are well known in the art, and therefore will not be discussed extensively herein. However, a brief overview might be helpful. Quadrature processing through the use of homodyne circuitry essentially is used to detect whether the outputs from a hydrophone represent a sonar reflection of a transmitted signal. The use of homodyne circuitry with a sinusoidal reference signal and integration over time effectively results in a bandpass transmission characteristic that is centered at the reference signal frequency and has a bandwidth that is inversely proportional to time duration covered by the integration. What this means is that for uncoded transmission waveforms, a longer transmission results in a narrower bandpass. However, range resolution for uncoded transmission is about one-half of the transmission length; i.e., resolution decreases with transmission length.
Prior art homodyne circuits have utilized analog processing circuitry, and are thus subject to undesirable characteristics associated with analog circuitry. For example, analog homodyne circuits are subject to drift resulting in undesirable, but inevitable, inaccuracies. Further, if prior art analog homodyne circuits are used in parallel, such as in quadrature processing, all parallel homodyne circuits must be matched in terms of performance characteristics and stability. Of course, the inherent drift and variability of analog homodyne circuits necessitates design that compensates for drift and variation.
A critical disadvantage prior art analog homodyne circuits is the delay associated with the processing of analog information, particularly the delay of analog-to-digital conversion of quadrature signals. That processing delay associated with analog circuitry eliminates the possibility of any real-time processing, particularly the decoding of coded waveforms. Of course, analog homodyne circuitry cannot be used for decoding overlapping waveforms.
The processing delay of analog homodyne circuits and the resulting inability to decode coded waveforms precludes improvement of resolution through the use of coded waveforms. As indicated previously, for uncoded waveforms range resolution is inversely proportional to transmission length. Decreasing transmission length would increase range resolution, but would deleteriously effect the effective bandwidth of the homodyne processing circuitry.
The processing delay associated with analog homodyne circuits further precludes continuous processing of the output from a hydrophone.
It is therefore an object of the disclosed invention to provide a homodyne processing system which advantageously utilizes digital circuit components.
Another object of the invention is to provide a homodyne processing system for generating digital outputs that can be analyzed in real-time.
A further object of the invention is to provide a digital homodyne processing system wherein coded waveforms are decoded in real-time.
Still another object of the invention is to provide a homodyne processing system having improved range resolution.
Yet another object of the invention is to provide a homodyne processing system which is capable of continuously examining a hydrophone output and does not utilize range gating.